1. Field of the Invention
The present invention relates to electrokinetic decontamination of soil, and more particularly to the rapid transfer of contaminants from the soil to a porous medium, from which the contaminants are easily retrieved afterwards.
2. Brief Description of the Prior Art
An electric potential, when applied to a soil through anode(s) and cathode(s), induces a migration of ions in addition to an electro-osmotic flow. Such an electro-osmotic flow displaces any contaminant existing in the pore fluid of a soil. If the contaminants are present in the pore fluid as ions, these ions will also move due to the electric potential without the need of any electro-osmotic flow.
When an electric potential is applied to a soil containing water, through electrodes, the molecules of water are dissociated by electrolysis into H.sup.+ ions and OH.sup.- ions. In response to the electric potential, the H.sup.+ ions produced at the anode will move into the soil, thus reducing the pH of the soil and enhancing the solubilization of some contaminants, like heavy metals for example. On the other hand, the OH.sup.- ions will also move into the soil thus increasing the pH of this soil in the vicinity of the cathode and thus enhancing the precipitation of some contaminants, like heavy metals for example. Obviously, this precipitation of certain contaminants into the soil in the vicinity of the cathode restricts the extraction of these contaminants out of the soil. For that reason, most prior art patents addressing the electrokinetic decontamination of soil teach the control of the pH at the cathode by circulation of some acidic solutions in order to avoid the pH reduction and the accumulation of certain contaminants into the soil in the vicinity of the cathode.
In most situations, electro-osmotic flow moves from the anode(s) to the cathode(s). In the case of contaminants existing as cations in the fluid, like heavy metals for example, the ionic migration caused by the electric potential will progress from the anode(s) to the cathode(s). In most situations, soil decontamination will progress from the anode(s) to the cathode(s).
For a given volume of soil to be treated by an electrokinetic process, the time and the electric energy required for the decontamination are proportional to the distance between the anode(s) and the cathode(s).
However, the progress of the decontamination of the soil between the anode(s) and the cathode(s) is not proportional to the treatment time and to the applied electric energy. The solubilization by acidification and the decontamination first progress rapidly in the vicinity of the anode. This initial decontamination in the anodic zone is, for a large part, a transfer of the contaminant to the vicinity of the cathode. Once the anodic zone of the soil is decontaminated, a relatively long time and a relatively large quantity of electric energy are required to acidify and decontaminate the cathodic zone of the soil.
Most of the prior art patents in the field of electrokinetic decontamination of soil recover the contaminant at the electrodes and thus count on the migration of the contaminants from one electrode to the other, or count on the decontamination of all the solid medium existing between the anode(s) and the cathode(s). The electrodes are either inserted in the ground (see for example U.S. Pat. Nos. 4,758,318 (Yoshida); 5,098,538 (Kim et al.); 5,190,628 (Bibler); etc.)) for in-situ treatment or placed at the ends of large cells for ex-situ treatment (see for example U.S. Pat. No. 5,137,608 (Acar et al.)).
More recently, U.S. Pat. No. 5,405,509 (Lomasney et al.) has proposed decontamination of soil using an electropotential gradient inducing migration of a target ion and immobilization and/or confinement of this target ion by an ion-permeable host receptor matrix. In this patent like in all cases of prior art, the feasibility of the treatment calls for a certain distance of soil between the anode(s) and the cathode(s). Decontamination of such a thick layer of soil is time-consuming and requires a large quantity of energy. In the prior art, the contaminants are extracted at the electrodes and must travel or migrate through the soil over a fairly large distance. This reduces the efficiency of the decontamination method.